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Abstract: The micellar self-assembly behaviour of a near-monodisperse linear poly(styrene-b-hydrogenated isoprene) (PS-PEP) diblock copolymer is examined in n-alkanes. Direct dissolution leads to formation of polydisperse colloidal aggregates that are kinetically frozen artefacts of the solid-state morphology. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) studies indicate that heating such copolymer dispersions up to 90 - 110oC gives well-defined spherical micelles that persist on cooling to 20oC. These observations are also consistent with small-angle X-ray scattering (SAXS) studies, which indicate the formation of star-like micelles in n-heptane and n-dodecane following a thermal cycle. Variable temperature 1H NMR studies in deuterated n-alkanes confirm partial solvation of the polystyrene micelle cores occurs on heating. Increased mobility of the core-forming polystyrene chains is consistent with the evolution in morphology via exchange of individual copolymer chains, as observed by DLS. Adsorption of this diblock copolymer onto a model colloidal substrate (carbon black) has been investigated using X-ray photoelectron spectroscopy (XPS). A Langmuir-type adsorption isotherm has been constructed using a supernatant depletion assay based on the aromatic chromophore in the polystyrene block. Comparable results were obtained using thermogravimetric analysis (TGA) to directly determine adsorbed amounts. Based on maximum adsorbed amounts at 20oC, these studies strongly suggest that individual copolymer chains adsorb onto carbon black from chloroform (a non-selective solvent), whereas micellar adsorption occurs from n-alkanes. This is important, because such copolymers are used as soot dispersants for engine oils. A near-monodisperse PS-PEP star diblock copolymer is examined in n-alkanes. Variable temperature 1H NMR studies using deuterated n-dodecane confirm that the outer polystyrene blocks are only partially solvated at 25oC, and solvation remains essentially constant on heating to 100oC. Physical adsorption of this copolymer onto carbon black is examined, with particular attention being paid to the effect of copolymer concentration on colloidal stability. Isotherms are constructed for copolymer adsorption onto carbon black at 20oC using a supernatant depletion assay based on UV spectroscopy analysis of the polystyrene aromatic chromophore. In addition, TGA is used to directly determine the amount of adsorbed copolymer on carbon black. Analytical centrifugation, optical microscopy (OM) and TEM studies indicate that the star copolymer acts as a flocculant for the carbon black particles at low concentration, with steric stabilisation observed above a certain solvent-dependent critical copolymer concentration. This is attributed to the spatial location of the polystyrene block, which enables copolymer adsorption onto multiple carbon black particles at low coverage, whereas all polystyrene ‘stickers’ adsorb onto single carbon black particles at high coverage, leading to steric stabilisation. SAXS is used to characterise copolymer-coated carbon black particles, providing complementary insights regarding changes in the fractal morphology that occur with increasing copolymer concentration. Moreover, SAXS also provided direct evidence for the presence of the copolymer chains at the particle surface. The effect of copolymer composition on both micelle diameter and dispersant performance (for carbon black particles) has been assessed for PS-PEP and poly(styrene-b-hydrogenated butadiene) diblock copolymers in n-dodecane. Direct dissolution at 20oC produces kinetically-frozen polydisperse aggregates, and higher polystyrene contents accentuate such an effect.

Open Access

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Abstract: The solution behavior of a polystyrene–hydrogenated polyisoprene star diblock copolymer (Mn ∼ 384 K; 6 mol % polystyrene) is examined in nonpolar media. Variable temperature 1H NMR studies using deuterated n-dodecane confirm that the outer polystyrene blocks are only partially solvated in n-dodecane at 25 °C: the apparent polystyrene content of 3.2 ± 0.2 mol % remains essentially constant on heating up to 100 °C. Physical adsorption of this star diblock copolymer onto carbon black particles is examined, with particular attention being paid to the effect of copolymer concentration on colloidal stability. An isotherm is constructed for copolymer adsorption onto carbon black from n-dodecane at 20 °C using a supernatant depletion assay based on UV spectroscopy analysis of the aromatic chromophore in the polystyrene block. Langmuir-type adsorption is observed with a maximum adsorbed amount, Γ, of ∼2.2 ± 0.1 mg m–2. In addition, thermogravimetric analysis is used to directly determine the amount of adsorbed copolymer on the carbon black particles, which are essentially incombustible under an inert atmosphere. Analytical centrifugation, optical microscopy, and transmission electron microscopy studies indicate that the star diblock copolymer acts as an effective flocculant at low concentration, with steric stabilization only being observed above a certain critical copolymer concentration (∼5.5% w/w based on carbon black). This is attributed to the spatial location of the polystyrene block and the star copolymer architecture, which enables copolymer adsorption onto multiple carbon black particles at low coverage, leading to bridging flocculation. Above 5.5% w/w copolymer, the surface coverage is sufficiently high that all of the polystyrene “stickers” adsorb onto single carbon black particles, resulting in colloidally stable, sterically stabilized carbon black dispersions. Small-angle X-ray scattering (SAXS) is also used to characterize the copolymer-coated carbon black particles: this technique provides useful complementary insights regarding the rather subtle changes in the fractal morphology that occur with increasing copolymer concentration. Moreover, SAXS also provides direct evidence for the presence of the copolymer chains at the particle surface.